Physics is suppoedly (from what i heard) the hardest of all a levels.
The majority of med schools will require chemistry through a level, bio usually as a supplemet up to a2, and then they'll state 'one more science' which usually means maths or physics, i think physics is preferred.
Biology is more worth doing then geog.
Chemistry is quite mathsy, one module in each year is pretty much based on equations (this is for OCR 2000 syllabus, which i'm doing now, but doubt it will change much). Biology is a bit repetetive, i dont think amazingly so, Chemistry is more repetitive because you have to redo all the organics from AS when going into A2.